Animal heart valves, such as porcine heart valves, have been used for implantation in humans. To prepare the animal heart valve for implantation, it is common practice to tan the valve. Tanning renders the animal tissue relatively inert with respect to the living host environment and brings about fixation of the tissue, i.e., stabilization of the tissue so that it has a fixed configuration.
As disclosed in Hancock et al U.S. Pat. Nos. 3,966,401 and 4,050,893 and Angell et al U.S. Pat. No. 3,983,581, it is known to tan animal heart valves by utilizing a tanning fluid under pressures which may range from 20 mm Hg to 120 mm Hg. These methods provide a substantial internal pressure on the heart valve. According to Hancock et al U.S. Pat. No. 4,050,893, the use of pressures in the physiologic range, i.e., 80-120 mm Hg causes the tissue to assume substantially its natural configuration during the tanning process and in fixing the valve in its natural contour so that it will retain its shape following the tanning process.
Recent reports by Broom, Neil D., An In Vitro Study of Mechanical Fatigue in Glutaraldehyde-Treated Porcine Aortic Valve Tissue, Biomaterials, Volume 1, Jan. 3, 1980, and Broom, Neil D. and Thomson, Fergus J., Influence of Fixation Conditions on the Performance of Glutaraldehyde-Treated Porcine Aortic Valves: Towards A More Scientific Basis, Thorax, Volume 34, 1979, describe problems that occur in the structure of the valve leaflets during tanning under pressures at the physiological levels. Briefly, the valve tissue of a porcine aortic valve includes collagen fibers arranged in a waveform or crimped condition. When internal pressures are applied to the valve during the tanning process, the collagen waveform tends to straighten out, and the degree to which the waveform returns to its original condition following removal of the internal pressure is a function of the magnitude of the internal pressure applied during the tanning process. If the collagen does not return to its original waveform condition, the stiffness of the valve leaflet is adversely affected. This has an adverse influence on the manner in which the valve leaflets open, and it provides regions of local strain or kinks in the valve leaflets during opening. It is believed that loss of the waveform geometry also reduces the fatigue durability of the valve leaflets.
According to these publications, obtaining fixation at the higher internal pressure levels results in considerable permanent loss of the collagen waveform so that, upon removal of the relatively high internal pressure, much of the collagen does not return to its original waveform condition. It is further reported by the same authors that a pressure of 100 mm Hg eliminates the waveform geometry entirely and that pressures even as low as 4 mm Hg result in significant reductions in the waveform after removal of the internal pressure.
To avoid these undesirable results, it is suggested in the above-noted publications that the tanning process should be carried out at pressures in the range of slightly above zero to about 4 mm Hg. However, an inherent effect of subjecting the valve to the tanning fluid is that the valve tends to shrink and distort. The relatively low internal pressure of just above zero to about 4 mm Hg is insufficient to retain the animal heart valve against shrinkage and distortion during the tanning process. Accordingly, the valve may become fixed in a configuration which is unsuited for use in an implant.
One prior art method for tanning at essentially zero pressure differential across the valve leaflet is to pack the valve leaflet with a batting, such as a Dacron or cotton batting, to assure that the leaflets contact each other to provide a competent valve. However, great care must be taken in packing the delicate valve leaflets, and even with the exercise of considerable care and diligence in packing, the mechanical stresses exerted on the valve leaflets cannot be measured and are subject to operator variability. In addition, abrasion between the valve leaflets and the batting results in changes in surface morphology, and a substantial amount of time is required to carefully pack the valve leaflets with the batting.